Can you fit a four wheeler in a 5.5 foot box without having to put the tailgate down? That would be my big factor in deciding to get a 5.5 foot or a 6.5 foot box.
In a year or two when the old ext cab craps out and I purchase a crew cab I'll definitely have to find one with a 6.5 box. I know I can tow a trailer but I'm not about rocks and debris flying up into the four wheeler.
You can fit a 7' fishing rod in a 5.5' box.
My two cents, I have a ram. There really nice trucks if you get a 2012 or newer. I have the v6 and get 23mpg around here, and the biggest thing I tow is a 17.5' boat and it pulled that to leech and back just fine. Got 14mpg doing it to. And there cheaper then Chevy/Ford.
My Yamaha Grizzly 700 fits in my 99 Silverado Ext. Cab with the tail gate up. I don't know how big of a box that is, maybe 6.5?My ATV doesn't even fit in my 6.5 box without the tailgate down. It is nothing big either. Can am 500.